As a follow-up to my sibling comment https://news.ycombinator.com/item?id=18410528: if your point is that the question should parallel the answer, note that the exchange could also go ”Who robs banks?” ”John Dillinger robs banks.” A question that can only be asked if you already know its answer (so as to decide whether the verb in the question should match a singular or plural noun) is not so useful!